CAT C15 08 Freightliner battery discharge protection buzzer!

What does the voltmeter show before, during, and after these events?
What does the voltmeter show on average while running, and on average after being shut down for a few hours?
When the truck batteries go bad, it is "usually" one cell in one battery, which will drain the remaining cells and batteries. Starting about this time of year when the temperatures start falling at night is when the batteries usually start showing their weakness, the colder the temps the more weak cells will show up.

 

Is the high twelves after the truck has not been running for a few hours?

 

There are a lot of guys that pass through this forum, so someone will have the solution. I would start by checking all battery connections, especially the one at the frame where the ground cables from the ground terminals of the battery hook to. The copper lugs on the cables connected to the steel bolt will sometimes look like a good clean connection, but when you remove the individual cable lugs and look at them, they will show corrosion from the two different metals touching each other, I usually scrape the mating surfaces with a pocket knife before assembly. I don't know what the triggering mechanism is for the low voltage system, whether it senses voltage from the batteries themselves, or whether the ECM perfoms this function. Another thing comes to mind, trace the battery cables from the batteries up to the starter, it could be that a cable has been rubbing something, and is just now through the cable insulation and starting to short out, triggering the low voltage.

 

Another thought, check the smaller wire, (probably about a ten gage wire), that is hooked to the "hot" cable stud on the starter solenoid, this wire feeds most of the cab, make sure that the connector lug on it looks like a good connection. Make sure that it hasn't been rubbing somewhere also.